Acoustic and optical wave chemical sensors are relatively new forms of chemical sensors which are based on transducers that monitor physical changes in a coating material or the surface of the transducer as material is sorbed either onto the surface of a material or into the bulk material. In acoustic wave device, when the wave is confined to the surface of the device, the sensor is characterized as a surface acoustic wave sensor and when the bulk material is vibrating, the is sensor characterized as a bulk acoustic wave sensor.
Acoustic wave sensors have been described by J. Grate et al in Analytical Chemistry, 1993, Vol. 65, No. 22, pp. 987A-995A, and in Analytical Chemistry, 1993, Vol. 65, No. 21, pp. 940A-948A, herein incorporated by reference. In the systems, the detector vibrates at a predetermined frequency and the frequency changes as material is sorbed. Acoustic wave chemical sensors have been shown to work in a variety of sensing environments and applications including detecting gases, vapors and selected species within gases and liquids. An example of an acoustic wave sensor is a surface acoustic wave (SAW) sensor.
A two-port SAW sensor includes an input transducer for generating an acoustic wave, an interaction or active region in which the propagating wave interacts with the environment, and an output transducer for detecting the wave. The acoustic wave characteristics can be altered by changes in material on or near the device surface. The cumulative effects of such an interaction over the propagation path of the acoustic wave result in changes in wave velocity, wave amplitude, wave frequency and phase delay at the output transducer. In the simplest cases, acoustic wave devices function as highly sensitive detectors of changes in surface mass, responding primarily to accumulated mass per unit area. Specific sensors are made by coating a film capable of sorbing a particular species or class of species from the environment to the interaction region of the device.
A number of prior art patents and articles describe acoustic chemical sensor devices. For example, U.S. Pat. No. 5,235,235, the subject matter of which is incorporated herein by reference, describes a multi-frequency acoustic wave sensor device for chemical sensing in both gas and liquid phases. The sensor detects changes in the surface wave at several different frequencies to obtain the sensor response.
J. W. Grate and R. A. McGill in Analytical Chemistry, 1995, Vol. 67, No. 21, pp. 4015 to 4019, the subject matter of which is incorporated herein by reference, describe making chemical acoustic wave detectors by applying a thin polymer film to a surface acoustic wave device. The authors also describe how certain polymers fail to wet the surface of the SAW and how certain polymers, once coated on the surface as a film, will dewet. The dewetting process results in separation of the polymer from the surface and the formation of globular polymer structures on the SAW surface.
Polymer coated SAW devices have been used for the detection and monitoring of gases and vapors in gaseous atmospheres. R. Andrew McGill et al. in Surface and Interfacial Properties of Surface Acoustic Wave Gas Sensors, ACS Symposium Series 561, Chap. 24, pp. 280-294, the subject matter of which is incorporated herein by reference, describe some of the problems of a conventional surface acoustic wave gas sensor wherein the SAW device has a polymer film coating.
A major problem of conventional chemical sensor devices is the interfacial sorption of water vapor at the device substrate-coating interface. This sorption of water vapor is a particular problem with quartz, presently the most popular piezoelectric material, because the quartz surface has silanol and silyl ether moieties which interact strongly with the water vapor. Even if the surface of the piezoelectric material is passivated with a non-metallic oxide, metal oxide, non-metallic nitride, or a metal nitride such as silicon oxide, aluminum oxide, silicon nitride, or aluminum nitride, the surface will still be populated with dipolar functional groups such as silanol or aluminum hydroxide which will significantly sorb water or other dipolar molecules. The adsorption of water vapor by the silanol groups on the quartz surface leads to corrosion issues and signal anomalies and difficulty in quantifying the response from the SAW and like device.
A number of different attempts have been made to eliminate the anomalous interference caused by the water vapor absorption. McGill et al. in Surface and Interfacial Properties of Surface Acoustic Wave Gas Sensors, supra, describe attempts to overcome the interference caused by adsorption of water vapor onto the quartz surface by plasma cleaning of the quartz surface followed by chemical modification of the quartz piezoelectric surface. The quartz surface was chemically modified by silanization of the surface with diphenyl tetramethyldisilazane, followed by coating the silanized surface with polyisobutylene, a polymer that selectively sorbs different chemical compounds. While this treatment partially overcomes the water vapor adsorption problem in that the silanized surface is superior to an unsilanized surface, the silanized surface still exhibits and anomalous results when exposed to water vapor. Because of this deleterious effect of water vapor on the SAW and like devices, the chemical sensors are not fully optimized.
Laser deposition of thin films is described by D. B. Chrisey and G. K. Hubler, Editors, in Pulsed Laser Deposition of Thin Films (Wiley, New York 1994), herein incorporated by reference.